1. Field of the Invention
This invention relates to electronic signal modulation, and more specifically, to spread spectrum modulation.
2. Description of Related Art
Spread spectrum techniques of communicating a message from a transmit unit to a receive unit are those modulation techniques which require a transmission bandwidth that far exceeds the message information bandwidth. The spread spectrum modulation characteristics should not depend upon the individual message to be transmitted as is the case with some other wideband modulation schemes such as wideband FM. There are many spread spectrum mechanisms. They can be conveniently classified as: (i) direct sequence, (ii) frequency hopping, (iii) time hopping, and (iv) hybrids.
In direct sequence spread spectrum modulation, a relatively narrowband message is spread with a `spreading code` to yield an encoded wideband signal. The spreading code is then required `despread` the signal at the receive unit.
There are many uses for direct sequence spread spectrum techniques. One primary use is that of spectrum sharing. It is possible for a number of different communicating pairs to occupy the same bandwidth simultaneously without significant mutual interference. This is usually accomplished in direct sequence spread spectrum systems by assigning each communicating pair a different spectrum spreading code.
Typically, one problem with direct sequence spread spectrum communications is synchronization. In order to function properly, the receiver must generate a copy of the spreading code at the same rate as the transmit unit. The receive unit must also correct relative phase discrepancies between the transmit and receive unit. This requires correct synchronization to be established and maintained at a stringent tolerance. Further, the receiver must know the particular spreading code employed by the transmit unit in order to despread the signal and recover the message.
With the increase of digital communication, more data is required to be passed over existing channels. This results in a need for increased data capacity per channel bandwidth.
Currently there is a need for a simplified method of increased capacity direct spread spectrum modulation of information.